The solution is to edit your registry. Make sure you are confident before you go anywhere near the registry because it's easy to screw up your computer if you do something bad. If you are confident - make a system restore port and backup your registry and then do the following:
Find the key under:
and modify the key from it's reference to CS4 (in my case) to CS6.
Exit and hey presto... the CS6 version should now open for your image file types.
I'm not sure whether this problem is Adobe doing something bad or whether the problem was caused by my registry cleaner ... I guess I'll find out next time it does another clean.